Happiness, they say, is infectious. Perhaps that is why the most popular course ever to be taught at Yale University—this semester enrolling 1,200 students, or a quarter of the undergraduate student body—is one titled “Psychology and the Good Life.”

PSYC 157, or “Laurie Santos’ happiness class” as it is affectionately known on Yale’s campus, teaches practical advice such as how to pick a meaningful career and how to separate satisfying pursuits from hollow ones. And now, an...